The ferric sulfate, used mainly in water treatment and stabilization of industrial effluents, is obtained commercially, usually in its solid state (approximately 80-60% of soluble ferric ion), or in aqueous solutions containing ferric ion up to 240 g/L. Commercial production processes vary depending on the availability of raw materials and their economic acceptability. The method that accounts for the highest production of the reagent worldwide is based on the oxidization of ferrous sulfate solutions, the main byproduct of the titanium and aluminum industry. The ferrous sulfate solutions are oxidized by hydrogen peroxide, with ferric sulfate later crystallized by evaporation from these ferric solutions (W. Büchner, R. Schiliebs, G. Winter and KB Büchel “Industrial Inorganic Chemistry”, VCH, Publisher, 1989, pages 525-526). A second industrial method is based on the oxidization and dissolution of magnetite, Fe3O4 (Chilean Patent No. 45508, 2009). In this method, the magnetite is attacked under pressure in glazed autoclaves with concentrated sulfuric acid, followed by oxidization of the ferrous ion content in the acid solution with hydrogen peroxide—this given that 24.1% of the iron present in magnetite is in ferrous iron form. A similar patent (U.S. Patent Application U.S. 20070048213) considers iron oxides as raw material, which is treated with sulfuric acid in an autoclave at high temperature and pressure. Other patents and publications describing the obtainment of polymeric ferric sulfate in aqueous solution consider ferric oxides and ferrous sulfate as raw materials (U.S. Pat. No. 7,387,770), using oxygen or nitrogen oxides as oxidizing agents (Fengting Li et al, Journal of Chemical Technology & Biotechnology, Vol. 68, (2) pp 219-221 (1997)), or sulfur dioxide and ferrous sulfate (Maohong Fan et al, International Journal of Environmental Technology and Management, Vol 2 (4), pp 393-401, (2002)). Moreover, various steel processing industries and galvanic industries employ an acid discard solution called “pickling” from which it is possible to obtain ferrous sulfate heptahydrate as a byproduct on a large scale, which, dissolved in water, is then oxidized with hydrogen peroxide in an acid environment. Another method, applied since ancient times and on a smaller scale for commercial production of commercial ferric sulfate, is the dissolution of metallic iron contained in iron scrap, which is treated with sulfuric acid, obtaining gaseous hydrogen and ferrous sulfate solution as a result; this ferrous solution is generally oxidized with hydrogen peroxide.
As noted above, the invention disclosed in this document is based on the oxidization of ferrous sulfate solutions obtained during the leaching of fayalite slag. The ferrous solutions obtained during this process also have significant concentrations of silicon (in the range of 1 to 15 g/L), as well as aluminum and arsenic, among other impurities in the slag, which are simultaneously and partially released during the process. In addition to the aforementioned impurities there are those in the leach solution fed to the process, for example when employing industrial acid effluents.
Silica (SiO2) contained in the ferrous acid solutions used in slag leaching, is presented in the form of a colloid composed of microscopic and sub-micron particles, which have, in general, meta-stable behavior at pH below 2.4. Physicochemical studies on destabilization of colloidal solutions of silica, and their subsequent precipitation, are based on the electrostatic decompression of the electrically charged colloid surface caused by the addition of trivalent ions such as Al3+ and Fe3+, or also of divalent ions such as Mg2+ and Zn2+ in a defined pH range. The coagulation of colloidal silica can be increased by the addition of coagulating agents and commercial flocculants. In the industry, the coagulation of silica from moderately acidic aqueous solutions by the addition of specific chemical agents such as Al3+, Mg2+ ions, silica or “activated sand”, followed by flocculation, obtains a residual sludge with over 60% p/p solid content. In the absence of flocculating agents, the content of solids in the final pulp does not exceed 10% p/p.
The last stage of the process disclosed in this document is the catalyzed oxidization by microorganisms in a bioreactor of the clean ferrous sulfate solutions obtained during the fayalite-slag leaching and subsequent precipitation of silica and impurities. The bio-oxidization of ferrous solutions is known chiefly in the field of natural and induced bioleaching of sulfide ores, which generates mine water and acid leaching solutions respectively. This natural phenomenon occurs by the action of iron(II)-oxidizing microorganisms, which obtain the energy required by their metabolic processes from the oxidization of ferrous to ferric ions in the aqueous medium, (W J Ingledew. 1982. Thiobacillus ferrooxidans: The Bioenergetics of an acidophilic chemolithotroph. Biochimica et Biophysica Acta 683: 89-117.), according to the following chemical reaction:

Such organisms may include the following species: Acidithiobacillus ferrooxidans, Leptospirillum ferrooxidans and Leptospirillium ferriphilum. The Chilean Patent Application 935-2007 of Apr. 3, 2007, discloses a ferric solution production process that uses as raw material: magnetite, ores or ore concentrates which are partially dissolved in sulfuric acid, releasing the ferrous ion. The ferrous solution is bio-oxidized by the Leptospirillum ferrooxidans bacteria cultures in a bioreactor stirred at 30° C., containing magnetite slurry.
The invention presented in this application differs from the procedures mentioned above in that: i) the fayalite slag used as raw material is industrial waste without economic value, unlike magnetite, which is a high-priced commercial product; ii) during the purification of the ferrous solution obtained in the slag leaching stage, a commercial byproduct such as silica gel, is generated and iii) bio-oxidization of the purified ferrous solution is carried out in a bio-oxidization bioreactor, with immobilized biomass, of the “air-lift” type, that operates continuously with short residence times (5 hours for the bio-oxidization of solutions containing 25 g/L of ferrous ion), which represents an operational and economic advantage.
The present invention corresponds to a commercial ferric sulfate production process whose plant can be installed scaled to the requirements demanded by the application of the bio-produced ferric solution process, using fayalite slag generated in copper-smelting plants. No previous process has established as a method of industrial application the use of this smelter slag in the bio-production of ferric sulfate solutions with ferric iron concentrations greater than 20 g/L including in the invented process a stage of acid slag leaching in dynamic heaps with control over the resulting silica and subsequent precipitation of colloidal silica and other impurities in a stirred reactor. The ferrous solution, free of colloidal silica and other impurities, is subjected to a process of bio-oxidization of the clean ferrous solution by microorganisms adapted to these metallurgical solutions.